Method of obtaining vacuum tightness in cast metal structures of thin section and resulting products



Jan- 28, 1958 M. J. zuNlcK ETAL 21,650 METHOD O INING VACU TIGHTNESS C ME STRUCT S THIN SECT AND RESUL G DUCTS Filed Sept. 28,

MICHAEL J. ZUNICK JOHN E. ILLINGWORTH BYI ' ATTORNEY United. States.,

METHOD OF OBTAINING VACUUM-TIGHTNESS IN CAST METAL STRUCTURES F THIN SEC- TION AND RESULTING PRGDUCTS 2 Claims. (Cl. 313-330) Thepresent invention relates in general to' vacuum sealing, and has more particular reference tomeans for.

and method, of Linsuring vacuum tightness at relatively thin sections in metal castings, the invention more especiallypertaining to the assurance of vacuum tightness in the evacuated envelopes of electron flow devices, such aslXeray generating tubes, more particularly at thin Wall sections of cast metal comprising portions of the anode of the ow device and `forming portions of its evacuated envelope.

An X-ray generator tube comprises an electron ilow device embodying an electron emitting cathode, an anode forming an electron target, and a sealed and evacuated envelope enclosing the anode and cathode in spaced apart facing relationship within the envelope. X-rays are produced in such a tube by applying electrical potential between the anode and cathode for electron driving purposes, in order to impel electrons, emitted at the cathode, toward and upon the electron target portions of the anode, at relatively high velocity, in order to constitute the anode target portions as an X-ray source. T he anode and cathode are commonly sealed in openings formed in the envelope, in fashion disposing portions of the anode and cathode structure in said openings, thereby constituting such portions as parts of the sealed and evacuated envelope.

Impingement of electrons on the anode target results in the generation of substantial quantities of heat which, especially in high powered tubes, tends to deteriorate and ultimately to destroy the target. In the interests of efficient X-ray production, the generator tube is usually operated under load conditions producing, in the target, high temperatures of the order of the melting temperature of the target material; and various expedients are employed for cooling the anode target in order to hold the same at a temperature, during operation of the tube, safely below the temperature at which the target would melt or become otherwise damaged or burned.

Water, oil and air are commonly employed as media for transferring heat away from the anode targets of X-ray generating tubes. Where air is employed as a cooling medium, it is common to provide the anode structure with an extension projecting outwardly of the envelope and carrying heat radiating tins or other heat dissip ating means so that anode heat may be conducted from the target, through the body of the anode to the radiating iins, and dissipated thence to circumambient atmosphere outwardly of the envelope. Such cooling by air is usually employed only in relatively low power generators, since air cooling is inherently incapable of dissipating heat at a suiciently high rate to accommodate the relatively large quantities of anode heat generated in high power tubes when in operation. Accordingly, for `high power tube cooling purposes, it is common to continuously deliver a cooling medium,` suchas-oil or water, through a chamber formed in the anode structure immediately behind-the target, the medium being circulated between the chamber and Yan 'external cooler to ist thus absorb heat from the target into the circulating medium and then extract the heat from the medium outwardly of the generator and dissipate it to atmosphere. The medium thus circulated in the chamber formed in the anode structure, immediately behind the target, is normally under pressure of the order of atmospheric pressure, as distinguished from the low pressure conditions within the evacuated envelope, so that the sectional thickness of the target portions of the anode structure, at said chamber, are required to be vacuum tight in order to preserve the evacuated condition within the generator envelope.

It is desirable, however, to make the said target portions as thin as possible, in the interests of rapid heat dissipation from the anode target to the circulating medium in the chamber. Anode structures, however, are commonly made of cast metal, usually copper, the target usually comprising a button of refractory material, such as tungsten, embedded in the copper anode structure adjacent the cooling chamber. It has been found necessary, in the past, to design X-ray tube anodes with walls of thicknesses of the order of not less than Ss of cast metal, behind the target button and between it and the cooling chamber, because of the relatively coarse granular structure obtained during the casting operation, which tends to open up along the grain boundaries of the constituent material during subsequent heat processing of the casting.

An important object of the present invention is to substantially reduce the sectional thickness in metal castings, as between the target button and the cooling chamber in an X-ray anode structure, while maintaining adequate vacuum tightness at such section of reduced thickness; a further object being to provide an X-ray anode structure embodying minimal wall thickness immediately behind the target button, in the interests of rapid heat dissipation.

Another important object is to accomplish minimal sectional thickness in cast metal structures, While maintaining vacuum tightness therein, by incorporating, in the structure, a relatively thin layer or sheet of relatively dense material, such as molybdenum, as a vacuum maintaining insert in the structure at the place or station Where vacuum tightness is desired.

Another important object is to incorporate, in a casting comprising a thin cast metal section, a thin plate or shell of dense, substantially impervious material, in order to render the thin cast metal section vacuum tight; a further object being to incorporate such shell or laye-r in the casting during the formation thereof as such, by applying the shell or plate as an insert in the casting mold and casting the metal into the mold in position to envelop and embed the insert in the cast metal.

Another important object is to provide an X-ray tube anode comprising a metal casting carrying a target button embedded therein at a relatively thin wall portion which defines a coolant circulating chamber in the structure, the casting incorporating a thin layer of molybdenum enveloped in the casting between the button and the cooling chamber.

The foregoing and numerous other objects, advantages and inherent functions ol' the invention will become apparent as the same is more fully understood from the following description which, taken in conjunction with the accompanying drawings, discloses preferred embodiv ments of the invention for the purpose of demonstrating the same.

Referring to the drawings:

Fig. l is a sectional view of an X-ray generating tube having an` anode structure of cast metal, having a thin walll section containing a vacuumA maintaining insert in accordance with ,thepresent invention;

Fig. 2 is an enlarged sectional view taken through the anode structure shown in Fig. 1;

Fig. 3 is a sectional view taken substantially along the line 3-3 in Fig. 2;

Fig. 4 is a sectional view taken through another anode structure embodying the invention;

Fig. 5 is a sectional view taken substantially along the line 5-5 in Fig. 4;

Fig. 6 is an exploded view illustrating the manner of mounting the pre-formed target button and the vacuum sealing shell or layer as inserts in an anode casting mold; and

Fig. 7 is a sectional view showing another anode structure embodying the invention.

To illustrate the invention, the drawings show an X- ray generating tube 11 comprising a cathode 12 embodying an electron emitting element 13, an anode 14 forming an electron target 1S, and a sealed evacuated envelope enclosing and supporting the anode and cathode in spaced apart facing relationship within the tube. While the invention is not necessarily limited to any particular form, construction or configuration of the envelope, or of the cathode 12, the envelope, as shown, may comprise a tubular glass element 16 having reentrant end portions 17 and 17', forming openings at the opposite ends of the envelope. These end portions may be circumferentially sealed respectively to the anode and cathode structures, in order to hermetically seal the envelope at said openings and to mechanically support the anode and cathode structures respectively upon the reentrant portions 17 and 17', so that the anode and cathode structures seal said openings and form parts of the envelope structure within the reentrant portions 17 and 17'.

The emitting element 13 may comprise a filament suitably supported on and insulated from the cathode structure 12, the filament being connected with lead conductors 18 adapted for connection with a suitable power source outwardly of the envelope, for the purpose of energizing the filament for electron emission. The lament may be connected with the lead conductors 1S through suitable envelope seals, preferably formed in the envelope forming parts of the cathode structure, ajt the inner end of the reentrant portion 17.

It will, of course, be understood that the generator tube 11 may be operated for the production of X-rays at the target 15, by energizing the filament 13 for electron emission, as lby connecting the conductors 1S with a suitable source of emitter energizing power, while simultaneously' applying electron driving potential between the cathode and the anode target, as by connecting the anode and one of the filament conductors 18 with a. suitable source of electron driving potential outwardly of the envelope. To this end, the necessary electrical connection with the anode may be made at or through the outwardly exposed envelope forming parts thereof disposed within :the reentrant portion 17.

Electrons emitted by the filament 13, when energized, may travel as an electron beam 19 from the filament 13 under the influence of the anode-cathode electron driving potential, and impinge upon the facing surface of .the target 15, thereby constituting the target as a source of X-rays which may be projected thence as an X-ray beam 20.

The present invention is not necessarily limited to any particular anode shape, style or configuration. The anode may comprise a body 21 formed of cast metal, such as copper, into which is set a target button 22 of suitable refractory target material, such as tungsten. Behind the target button 22, the body 21 may form a wall 21 defining a cavity 23 for the circulation of a cooling fiuid in heat exchange relationship with the button 22, through the wall 21', for the continuous cooling of -the target, the sectional thickness of the cast metal wall 21 behind the button 22 being of the order of g", vor less, which is ordinarily insufficient to provide vacuum tightness in a cast metal wall of such lthickness. In order to insure vacuum tightness in said relatively thin section of cast metal behind the button 22, a layer 24 of dense, irnpervious sheet material is embedded in the wall 21. As shown more particularly in Figs. l, 2 and 3, the layer 24 may comprise a cup-shaped element having a bottom wall 25 extending in the wall 21 immediately behind the target button 22, the vertical spacement between the facing surfaces of the button 22 and the wall 25y being of 'the order of 1/32, or less.

The member 24 preferably comprises molybdenum and has wall thickness of the order of from 0.005" to 0.010" so that the combined thickness of copper, molybdenum, and the material of the target 21, between the cathode facing surface of the target button and the chamber 23, is of the order of 1A; or less, of which distance the thickness of the target button accounts for about 1/16.

The anode structure, also, if desired, may be formed with staggered partition forming ns 26 and 26 extending in the chamber 23. These tins may be integral with the side and bottom walls of the cavity 23 and may include a pair of spaced apart lateral fins 26 extending from one side of the cavity toward the opposite side thereof, as shown more particularly in Fig. 3 of the drawings, and a central fin 26 spaced from and extending medially between said lateral fins from the opposite side of the cavity 23. These fins may serve to strengthen .the wall structure, and will also serve as deflecting baffles for cooling fiuid circulated in the chamber 23, as hereinafter more fully described.

The anode structure may also include a sleeve-like extension 27, one end of which may be brazed or otherwise sealingly secured on the casting 21, as at 28, in open communication with the cavity 23. The opposite or open end of the member 27 may be fitted with a cover member 29 carrying the outer end of a tube or pipe 30 in position extending outwardly of the cover member 29 for connection with a source of cooling fluid. The inner end of the pipe 30 may be sealed, as at 31, in an opening 32 formed in a preferably metal disk 33.

This disk may be sized to extend snugly Within the cavity 23 in position overlying the wall remote edges of the fins 26 and 26', the opening 32 being disposed opposite the wall remote end of the fin 26. Outwardly of the fins 26, the disk 33 may be formed with a pair of cut-out openings 34; and the end closure member 29 may be formed with an outlet opening 35. A suitable cooling fluid may be delivered, as by means of a pump or other circulating device, through the pipe 30 from a source of such fluid disposed outwardly of the X-ray generator tube. The cooling fluid may pass thence through the opening 32 into the cooling chamber 23. Such fluid may travel in the chamber 23 through the channels defined by and between the baflles 26 and 26 in heat exchange relationship with the target button through its supporting wall portions of the anode. The cooling fluid may be discharged from the chamber 23 through the openings 34 into the sleeve-like member 27, and may pass thence through the outlet opening 35 which, if desired, may be connected with the external source of cooling uid, preferably through suitable means for cooling the fluid for recirculation to the chamber 23.

In order to seal the anode in the envelope member 16, the body 21 of the anode may be formed with a peripheral shoulder 36 upon which is sealed, as by welding or brazing, a cup-shaped seal member 37 having annular skirt portions forming a lip adapted to form a circumferential glass-to-metal seal 38 with the reentrant envelope portion 17, at the inner end thereof. The anode may thus be sealingly mounted and supported on the envelope member 16 so that the seal member 37, as well as portions of the anode including the target supporting wall portions 21', may form portions of the envelope structure closing the opening formed at the inner end of the reentrant portion 17.

The present invention, of course,"i'sknot necessarily limited to the form and arrangement'of the parts of the anode structure. As shown more particularly in Figs. 4 and 5 of the drawings, the body 21 may be formed with a single medial partition wall 26 formed integral with the opposite side walls of the chamber 23 and dividing it into a pair of compartments 23 on opposite sides of the partition, said partition wall having an edge spaced from the bottom 25 of the cup-shaped member to denc a slot or opening 23 between said edge and the bottom of the cup. This slot serves to interconnect the compart ments 23 and to assure that the cooling medium, in flowing in the chamber from one compartment to the other, will pass immediately behind the target, in efficient heat exchange relation therewith.

The disk 33, which covers the open side of the chamber 23, in Figs, 4 and 5, may be formed with inlet and outlet openings 32 and 34 disposed therein in position respectively communicating with the compartments 23 on opposite sides of the Walls 26, the inlet pipe 3i) being sealed in the opening 32, at the inner end of said pipe. The disk 33 also may be sealed at its edges to the inner end of the sleeve-like extension 27, which, in turn, may be sealed in the body 21 as at 28, and a sealing skirt 37, making a glass-to-metal seal 38 with a reentrant glass envelope portion 17, may be sealed, as at 36, upon peripheral shoulder portions of the body 21.

An enclosing, preferably sheet metal, shell 40 may be applied upon the body 21 and secured in position, as by crimping the peripheral portions 41 of the shell into lool;- ing engagement in circumferential grooves 42 formed in the body 2l. The shell 40 may include skirt portions 43 extending around and enclosing the glass-metal seal 3S to protect the same from stray electron impingement. The shell may also be formed with portions 44 enclosing the target end of the anode, such portions being formed with an opening 4S in alinement with and between the electron emitting element of the cathode and the anode target formed by the button 22.

lt is, of course, also possible to incorporate structures embodying the present invention in other types of anodes, including anodes for use in X-ray generators of the sort shown in United States Letters Patent No. 2,356,645, issued August 22, 1944, on the invention of Z. I. Atlee and H. W. Brackney. Such an anode is shown in section, in Fig. 7, and may comprise a tubular metal sleeve 46, forming a portion of the envelope of the generator tube, a target structure 47 being sealed in the end of the tube in position to be bombarded by electrons emitted by a cathode enclosed in and supported by the tube envelope at the target remote end of the tube 46. The cathode is thus in position to emit an electron beam longitudinally through the tube 46 for impingement upon the target. As shown in Fig. 7, the target structure 47 may comprise a body 21 of cast metal, such as copper, in which a target button 22 of refractory material, such as tungsten, is set, the structure including a disk or plate 25 of dense, irnpervious material, such as molybdenum, enveloped in the copper'body of the target structure behind the target button 22, the body 21 in-cluding an outwardly extending peripheral flange 48 to facilitate the sealing of the target structure 47 in the end of the tube 46, as by welding or brazing the tube end to said flange.

Anode structures embodying the present invention, regardless of their shape or configuration, may be formed by placing the target button 22 and the member 25, whether formed as a cup or as a plate, as inserts in a suitable casting mold in which the inserts may be anchored. To this end, as shown more particularly in Fig. 6 of the drawings, the target button 22 may be anchored on a mounting plate 49, preferably comprising graphite, by means of molybdenum anchoring pins 50, said pins having ends secured in openings 51 in the anchor plate. The member 25, whether the same comprises a at plate as shown in Fig. 7, or the bottom of a cup-shaped member 6 of the sort shown in Figs. 3 and 4, may be anchored on the mounting plate 49 in stacked relationship with respect to the button 22, as by means of molybdenum anchoring pins 52, the same having ends adapted to be secured in openings 53 formed in the mounting plate 49, at the edge of the button 22 and intermediate the openings 51.

When the parts are so mounted, it will be seen that the member 25 is supported in spaced parallel relationship above the target'button 22., the vertical distance between the facing surfaces of the member 25 and the button 22 being equal to the diameter of the hooked disk retaining ends of the pins 50. The inserts 22 and 25, thus anchored on the mounting plate 49, may be fastened in a suitable mold shaped in conformity with the desired shape of the anode body 21, and copper may then be cast into the mold, upon the so mounted insert elements 22 and 25 in order to embed and envelop said elements in the casting in proper relative position therein, the button 22, in eect, being brazed to and upon the member 25, by the intervening cast material which enters between the space dened by the hooked portions of the anchor pins S0, between the facing surfaces of the elements 22 and 25. The casting operation is preferably accomplished under vacuum conditions. After the casting operation has been completed and the anode structure removed from the mold, the mounting disk 49 may be stripped from the casting and the projecting ends of the pins 50 and 52 may be removed, leaving portions of the anchoring members 50 and 52 embedded in the cast material in the edges of the target button.

It Will be seen from the foregoing that the present invention provides a novel method of insuring vacuum tightness in the metal casting comprising the thin wall section 21 which extends behind the target button 22 and between it and the chamber 23, such wall section comprising a part of the evacuated envelope of the X-ray tube. This is accomplished by embedding in said section a relatively thin layer of dense, impervious material, such as molybdenum. The resulting structure is not only vacuum tight at said thin wall section, but said wall section, being thin, allows for the rapid transfer of heat from the target button 22 to the cooling fluid in the chamber 23.

X-ray anodes incorporating the novel features of the present invention not only show improved and efncient anode cooling characteristics; but the incorporation of integral coolant guiding baflles, such as the bales 26 and 26', in the anode structure eliminates the necessity of assembling relatively more expensive conventional cooling coils in the chamber 23, the guiding baffles 26 and 26', together with the inexpensive cover disk 33, being a cost reducing improvement in uid cooled X-ray tube anodes.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the forms herein being preferred embodiments for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

l. An anode comprising a casting of cuprous material having a wall forming an electron target on one side thereof and defining a chamber for the circulation of a cooling medium on the opposite side of the wall, said casting including an integral n forming a partition dividing said chamber into compartments communicating one with another between an edge of the fin and said wall of the chamber.

2. An anode comprising a casting of cuprous material having a wall of relatively thin section forming an electron target on one side thereof and deinnig a chamber for the circulation of a cooling medium on the opposite side of the wall, and a thin layer of dense impervious material embedded in said wall and integrated therewith to render the same vacuum tight, said casting including an integral 7 'u forming a partition dividing said chamber into compartments communicating one with another between au edge of the fin and said Wall of the chamber.

References Cited in the le of this patent UNITED STATES PATENTS 1,994,140 McEuen Mar. 12, 1935 Marden et al. Oct. 29, 1929 

